Network analyst public transit tool Prepare time lapse polygons generate polygons along LineVariantElements rather than around stops

JennyRassmus

I have created a network dataset with the public transit tool. I ran the regular service area solver with an expected result with service areas around stops. I then ran the Prepare time lapse polygon tool, but the polygons appear strange to me. They are located along the LineVariantElements and don't have that large polygon in the city center. Also, in some places they spread out along the road network unexpectedly. 

Does anyone have an explanation to this behaviour? And what can I do to avoid it? Attached are a few screenshots with explaining text.

I use ArcGIS Pro 3.2.0.

PrepareTimeLapsePlygons.pdf

Melinda Morang

Hi Jenny.  I can help you with this.

I think the difference between your Service Area layer (the green one) and the output of the Prepare Time Lapse Polygons tool is due to the Service Area polygon detail level.  The Prepare Time Lapse Polygons tool defaults to high precision (detailed) polygons.  Your green polygon looks like standard precision to me.  Standard precision polygons generate faster but are less precise.  They don't generate holes in areas with unreached roads if those unreached roads are fully surrounded by reached roads.

Why do you have unreached roads?  Based on your screenshots, I think your network dataset has some connectivity problems.  In your green polygon, it looks strange to me that the stops further from the city center only have very small polygons around them and don't extend further into the street network.  Also, from the Prepare Time Lapse Polygons tool, it looks like a lot of the streets in the city center weren't reached, is another indication.  Network connectivity problems are very common and often cause behavior like this.

Some likely problems with connectivity include the following:

• Using the wrong connectivity policy (End Point instead of Any Vertex, depending on the configuration of your streets)
• Streets that have small gaps or dangles at intersections instead of being topologically connected
• Streets with no vertices or endpoints at intersections.

Please take a look at this section of our network dataset troubleshooting page for some further information and tips for diagnosing and fixing the problem: https://doc.esri.com/en/arcgis-pro/latest/help/analysis/networks/troubleshoot-network-analysis-problems.html#75D